Generally, a pair of binoculars includes various adjustment mechanisms such as a focusing mechanism, a diopter adjustment mechanism in addition to a pair of right and left lens barrels each including a telescopic optical system. With the diopter adjustment mechanism, diopter difference between right and left eyes of a user is adjusted. With this state, the user carries out focusing right and left telescopic optical systems by the focusing mechanism. After adjusting right and left diopter difference, even if the object to be observed is changed, the user can observe the object by just carrying out focusing unless the user is changed.
When the object to be observed is a fixed point locating near to infinity, or when the object to be observed is locating far away so as to be able to focus the object by means of accommodation of the user's eyes, the object is preferably observed by a fixed focus depending upon accommodation of naked eye or depth of field. Moreover, after adjusting focus, it is preferable that focusing is fixed, as a matter of course, during observation, and even in a state that a distance to the object is not near to infinity (see Japanese Utility Model Publication No. 2592717).
However, when an operation portion of the diopter adjustment mechanism that has already been adjusted is touched by mistake during observation, there is a danger that diopter difference goes wrong. When diopter difference goes wrong, diopter difference of right and left eyes has to be adjusted again, so that the observation has to be interrupted. Moreover, when the operation portion of the focusing mechanism is touched upon using binoculars, focus position may be changed.